Fun with Light

There are lots of cool science activities you can do at home with light.

Like I’ve done almost every year of my life, I spent my summer break at my family bach at Oakura. Last summer I wrote a post about a trip to the rocks and what could be found living there. This summer, on the relatively few sunny days we had, I had fun playing with light.

Make a Telescope

The previous year, I made a simple telescope out of a $2 set of two magnifying glasses. Playing with trial and error and a piece of soft wood, I ended up with something that had a zoom of about 2x. However, because it only used two lenses the resulting image was inverted.

This summer, I came prepared with an extra set of magnifying glasses, making four in total. I raided the recycling bin and used some ginger beer bottles to hold them in place, facing an island in the bay. Then I moved them back and forth until the zoom and focus seemed as good as I could get it.

Once I had the placement right, I marked off the distances on a long piece of wood, then taped the magnifying glasses to it. What I ended up with wasn’t the strongest or most portable telescope in the world, but all it took to make was $4 and a fun afternoon.

See Shadows Jump

My brother Jeremy is a concept artist for Weta Workshop, which has left him with a good understanding of light and colour. One evening up at the beach he started talking about some interesting things that shadows do.

Watching shadows of leaves dance on the ground, he wondered if they often form natural pinholes. When we had a partial solar eclipse in Auckland in 2012, my mum (who also has a great artistic understanding of light and colour) mentioned to me how the shadows in her garden looked strange when she went outside during the eclipse. This would have been due to the pinhole effect, and it’s why some of the recommended ways of viewing an eclipse are to make a pinhole in a piece of paper or use a colander.

You’ve probably seen diagrams showing the basics of how a pinhole camera works. Even without a lens, when light passes through a small hole it can project a sharp image on a surface opposite that hole. However, that image will be inverted (like in my first attempt at making a telescope).

I often collect pāua shells from my trips to the rocks when I’m at the beach. A pāua shell has a row of holes along one side. When I held it a certain distance away from a wall, with the Sun low on the horizon, we found it made a row of pinholes. But because a projection of the Sun looks the same inverted as it does normally, in order to tell if the image really was inverted I moved a cardboard roll behind the pāua and watched at the holes “filled up” with shadow backwards – just as we’d expected.

But something else happened which I definitely didn’t expect. Watch this video we took to see the shadow of the pāua shell reach out to touch the cardboard roll’s shadow as they get close together:

I understand why the circles fill up backwards, but why do the shadows jump together like that to start with? pic.twitter.com/G6Zv18LTpq

If instead the pāua shell was held closer to the Sun and the cardboard roll was closer to the wall, then we found it would be the shadow of the cardboard roll that bulged out as they got close.

We immediately took to pen and paper to try to draw out diagrams that would explain how this worked. My initial idea was that we were seeing the area of intersection between the penumbras – the hazy edge of the shadows where the Sun was only partially obscured. But this wouldn’t explain why the bulge would change depending on which object was in front of the other.

Before too long, one of Jeremy’s ray diagrams seemed to explain what was happening. I’ve tried to reproduce them here (I hope you’re all suitably awed by my skills with MS Paint):

This diagram shows a light source on the left casting a shadow from the object in the middle onto the surface on the right. It shows how a non-point light source such as the Sun produces a shadow with an umbra (where none of its light reaches) and a penumbra (where part of its light reaches). The darkest part of the shadow, the umbra, is the middle section between the lines on the right.

Now, what would happen if I insert another object partly between the light source and the first object?

The new object blocks some of the light from reaching the original object. As this ray diagram shows with the red line – where the light is partially blocked – the result of inserting this second object is that the umbra of the first object’s shadow is extended toward the new object. This is the cause of the bulge you can see in the video above.

It turns out this shadow jumping effect is called the shadow blister effect. You can observe it easily for yourself on any sunny day.

Wave at the International Space Station

The sky at Oakura is lovely and dark, with the nearest city being nearly 50 km away. Before the Moon rose one night after Christmas a few of us went up a nearby hill to stare up at the night sky.

With a clear dark sky, you can see the band of the Milky Way galaxy arc across the sky like a pale cloud, as well as the fuzzy blobs that are the Large Magellanic Cloud and Small Magellanic Cloud. These are dwarf galaxies which orbit the Milky Way.

We also saw many meteors, and a surprisingly high number of satellites. From Earth satellites look just like stars, except they move steadily across the sky in a straight line. Usually they appear quite dim, but there is one satellite in particular which can shine brighter than any star in the sky, and even brighter than any of the planets. That is the largest artificial satellite of them all: the International Space Station (ISS).

The ISS orbits the Earth about once every 90 minutes, and although it doesn’t pass over New Zealand each time it does fly over us more often than you might think. But we can’t always see it in the sky; the conditions have to be right first.

Before we can see the ISS the sky needs to be dark enough for it to stand out. Also, it needs to be in the right position for sunlight to reflect down at us off its massive arrays of solar panels. This means that you’ll only be able to see it in the hours after sunset and before sunrise.

It generally takes 1-6 minutes for the ISS to pass visibly overhead. This will usually end with it appearing to fade into darkness as it stops reflecting sunlight back at us – you won’t see it set over the horizon like you would with the Sun or Moon.

NASA has a great online service, which you can subscribe to and get email alerts, that can tell you when and where to look to spot the ISS. It’s called Spot The Station. It lets you enter a city, and will tell you when the next few ISS sightings will be as well as how long they will last, and how it will travel across the sky.

ISS sightings often come in clusters – there will be sightings around a similar time in the morning or evening for several days in a row, followed by a period of no sightings. If you’re extra lucky, you might get to see it twice in one evening as it comes back round an hour and a half later.

I’d be remiss if I didn’t also mention that you can rent our bach if you ever want to see Oakura with your own eyes.

4 Responses to “Fun with Light”

Totally agree with you Mark. Playing with light can be really interesting….. and fun.

My first telescope was a beginner’s refractor…. and the Universe came alive and has fascinated me ever since. My first microscope was a pair of binoculars turned around the wrong way and I found a whole new magnified world waiting to be discovered. I now make videomicroscopes for a living so that others can experience this micro world.

It was looking at the sky at Oakura through binoculars a couple of years ago, and seeing how many more stars there were that I couldn’t see with my naked eyes, that convinced me to buy a telescope. Also a beginner’s refractor, so possibly quite like yours!

I’ve contemplated from time to time getting a microscope of some description to take up there with me. Some things I find just *beg* for a closer look, I’d love to look at sand or pieces of crab exoskeleton for example. I’d thought the best thing I had up there for magnification was the cheap magnifying glasses I had, but hadn’t thought about trying binoculars for that.

I’ve occasionally seen people talking about pieces of kit that can effectively transform a smartphone camera into a microscope. Perhaps I should look into getting something like that this year.

Take the eyepiece out of your telescope’s drawtube, turn it around the other way and,hey presto, its a simple microscope! Not the world’s best microscope (working distance and the optics might be only average) but give it a try.

The name of this blog, Honest Universe, is derived from the idea that the universe in which we live is honest – if you ask the right questions in the right way you’ll eventually get the right answer. This is the premise upon which science is based. It also reflects Mark's philosophy of putting a high value on honesty and integrity.

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